Modems have been used to couple a data signal onto the power lines of an electrical power delivery system. An example of such modems has used a toroid one-turn transformer inserted around an overhead conductor of the power delivery system. By this method of magnetic coupling, the output current of the modem, consisting of the high frequency communication signal, induces a voltage of the same frequency and in series with the overhead conductor. Farther down the line, a second such toroid one-turn transformer may be inserted around the overhead conductor so that the high frequency communication signal is magnetically induced to appear across the terminals of the second toroid one-turn transformer. The signal is then processed to recover the data. The data can then be recoupled onto the line with the same coupler to pass the data to the next coupler/modem down the line. Through use of a large number of such couplers/modems, data can be transmitted a long distance over power distribution lines.
An advantage of the toroid one-turn transformer as a coupling device is that it can readily provide isolation between the high voltage overhead conductor and the modem. Further, such a transformer has bilateral properties meaning that the coupling coefficient from the modem to the line is equal to the coupling coefficient from the line to the modem. Notwithstanding these advantages, such toroidal transformers have a relatively low coupling coefficient and insufficient gain.
There is a need for a capacitive coupler which has a relatively high coupling coefficient and gain, and which is relatively inexpensive to manufacture, install and maintain.